The general theme of the sonnet is that what is written about in poetry is eternal - specifically in this poem, Shakespeare is admiring a woman, and saying that her beauty will never fade because he is putting it into verse. He begins by comparing her to a summer day, and then saying she is much more beautiful. He continues comparing the details of the summer day to his subject showing how she is much fairer. He ends the poem by focusing on the subject, her beauty, and her qualities which will be made eternal by the poet putting them into poetry.

The whole verse is Shakespeare about the verse itself surely? We are led to think it is intended for a loved one, but Shakespeare is declaring that his own creation will remain eternal, note how he spends more time describing the actual summer and not his intended. The beauty he is describing is the poem itself.

Nature fades but art is immortal. Though beautiful at moments in time, everything in nature enjoys but a moment of perfection. In time every virtue will be destroyed, every potential beauty ravaged by the elements, and every perfection will come to contain imperfections. In art, however, the essence of perfection will be captured. Though everything in the world dies and fades, the subject of poetry enjoys eternal life.

The above commentators rightly argue that ‘Sonnet 18’ is about the eternity of Shakespearean “lines”. This interpretation, however, can be extended a little further. The sonnet is not only about Shakespeare’s “eternal lines”, but it is also about how in time Shakespeare’s observations grow. In the final line of the third quatrain Shakespeare notes “in eternal lines to time thou gorw’st”. The beloved’s beauty not only remains unchanged in perpetuity, but it also grows parallel to time. In the couplet Shakespeare confirms this observation: “this gives life to thee”. Shakespeare’s observations will not only be read “so long as men can breathe”, but they will also offer life to the object which Shakespeare appreciates and this life giving force will give the object the potential to live in perpetuity. As the object grows eternally Shakespeare’s observations too receives perpetual growth from within—critics even today continuously enrich Shakespearean “eternal lines” with new interpretations.